1. Field of the Invention
This invention relates generally to spread spectrum communications and, more particularly, to a system and method for a mobile station receiver to efficiently recover system time after its reference clock has been disabled during a sleep interval.
2. Description of the Related Art
During the idle state, a direct sequence spread spectrum (DSSS) base station operates in slotted mode to reduce mobile station standby power consumption. The base station instructs receivers to be turned off for most of the time, and to wake up only periodically to monitor the paging channel for pages and network overhead messages. During the time the receiver is turned off, the mobile station receiver sleep control system can further reduce power consumption by operating with a low frequency sleep clock instead of the high frequency reference clock.
In a DSSS mobile station receiver, code sequences, such as pseudorandom noise (PN) or Gold code sequences, are used in order to maintain time synchronization with the base station. For example, the system compliant with the IS-2000 standard uses a short PN sequence of length 215 and a long PN sequence of length 242−1. After the mobile station receiver initially acquires the synchronization from the base station, the short and long PN sequences advance periodically based on the mobile station receiver reference clock. An automatic frequency control algorithm and voltage controlled local oscillator are used by the mobile station receiver to maintain a high degree of reference clock accuracy relative to the received (transmitted by the network) signal.
When the mobile station receiver operates in slotted mode and is not monitoring the paging channel, the mobile station receiver is turned off, along with its reference clock that is replaced by a low frequency sleep clock. During this inter-slot period with the reference clock turned off and only the sleep clock running, the short and long PN sequences stop advancing and their states are no longer synchronized to the short and long PN sequences of the base station. After the mobile station receiver wakes up, the states of the short and long PN sequences need to be adjusted to compensate for the loss of system timing during the inter-slot period. The mobile station receiver needs to correctly perform the adjustments before the arrival of its assigned time slot in order to resume receiving the paging channel messages.
Conventionally, the short and long PN sequence adjustments are made by permitting the reference clock to be turned on and off only at certain convenient points. The problem with this method is that the sleep interval becomes fixed to one, or just a few values, and the reference clock must be turned on well before the sleep interval ends to insure sufficient time for synchronization. Thus, the mobile station receiver power conservation is not maximized. The power conservation problem is worsened if the reference clock must be turned on even earlier to compensate for possible errors in the mechanism used to time the sleep interval. The sleep clock drift problem is discussed in greater detail below.
Typically, in a Code Division Multiple Access (CDMA) system, the mobile station receiver must periodically receive paging slots, as determined by a SLOT_CYCLE_INDEX value. The index is selected by the mobile station receiver, except that the network can set the maximum index.
In addition, there exists a certain amount of overhead to receive a slotted page message. Because of continuous convolutional coding on the CDMA paging channel, the mobile station receiver conventionally has to receive at least a frame before the slot, depending on the paging channel data rate. This time, in conjunction with various turn-on times in the mobile station receiver, results in additional overhead. The total duty cycle can be as much as approximately 15.6%, depending on the slot cycle length.
Furthermore, it is possible that the mobile station would be required to receive two paging channel slots. This can occur if the base station uses the MORE_PAGES bit in the SLOTTED PAGE MESSAGE, thereby requiring the mobile station to receive up to one additional slot. Also, the CDMA specification states that the mobile station may stop listening to the paging channel after reading the SLOTTED PAGE MESSAGE. There is no guarantee that this message is located at the beginning of the slot. As a result, it may happen that the mobile station must always listen to the entire slot.
One conventional power-up method reads the state of the long code generator just prior to powering down the mobile station receiver. A complex matrix multiply operation is then applied to the long code to determine the correct state of the long code generator at a time in the future when the long code generator is to be reinitialized.
However, this approach is computationally expensive. As a result, it may be necessary to “WAKE UP” the mobile station receiver earlier than would be necessary if the complex matrix multiply operation is performed after the power down period. If the matrix multiplication is performed before powering down, then the mobile station receiver must remain in a powered up state for a period of time sufficient to accomplish the matrix multiply. In either case, the mobile station receiver is powered on for a longer time. This causes the overall duty cycle and power consumption to increase, thus decreasing battery life.
Regardless of overhead time that must be used in synchronization, additional overhead is required due to calibration errors. The amount of adjustment applied to the short and long PN sequences is in theory a function of the number of reference clocks absent during the inter-slot period. However, this number is not known to the mobile station receiver and has to be estimated based on the relationship between the reference clock and the sleep clock. This relationship can be observed by the mobile station receiver while both clocks are running simultaneously. This relationship, however, varies in time due to external factors such as temperature. A poor estimate results in longer time for the mobile station receiver to perform timing re-acquisition after waking up. An incorrect estimate results in total loss of timing synchronization and the mobile station receiver will not be able to receive valid messages from the paging channel. When this happens, the mobile station receiver has to re-initialize by performing the initial timing acquisition. Both consequences have negative impacts on the mobile station receiver power consumption.
In order to derive an accurate estimate of the missing reference clocks during the inter-slot period, the mobile station receiver has to regularly calibrate the sleep clock frequency based on the reference clock. This sleep clock calibration method requires longer calibration periods and is inherently limited in the accuracy that can be obtained.
Another solution to the problem of sleep clock drift is to use a larger search window when attempting to reacquire the state of the PN code received from the base station. However, the use of a larger search window typically requires a correspondingly longer search time, and results in poorer power conservation results.
It would be advantageous if additional power savings could be realized by a mobile station system during slotted sleep mode.
It would be advantageous if the frequency of the sleep clock could be accurately measured using the reference clock to reduce the resynchronization time.
It would be advantageous if actual frequencies of the reference and sleep clocks, as well as the frequency drift of the sleep clock, could be used as data to reduce the resynchrornzation time.